1. Field of the Invention
The present invention relates to an electrically operated brake including electric motors as a drive source for braking a moving body with a friction force, and an electrically operated braking system equipped with such an electrically operated brake.
2. Discussion of the Related Art
Published International Application WO96/03301 discloses one known example of an electrically operated brake using an electric motor. This electrically operated brake comprises (a) a friction member for braking a moving body, (b) an electric motor, (c) a planetary gear device including a sun gear, planetary gears meshing with the sun gear, a carrier rotatably supporting the planetary gears, and an internal or ring gear meshing with the planetary gears, and wherein the ring gear is a stationary member while the electric motor is connected to the sun gear, (d) an output shaft connected to and rotated with the carrier, and (e) a friction member driving device connected to the output shaft, for moving the friction member by rotation of the output shaft.
In this known electrically operated brake, the drive torque of the electric motor is boosted by the planetary gear device, and the boosted drive torque is transferred to the output shaft, so that the output shaft is capable of producing a braking torque which is larger than the drive torque produced by the electric motor.
However, the known electrically operated brake suffers from a problem of difficulty in constructing the brake so as to permit the output shaft to increase its braking torque. Described in detail, the braking torque to be produced by the output shaft can be increased by increasing the drive torque of the electric motor. In this case, the electric motor should have a large capacity and tends to have an undesirably large size(large diameter, in particular). Alternatively, the braking torque of the output shaft can be increased by increasing the speed reduction ratio of the planetary gear device. In this case, the rotating speed of the output shaft tends to be undesirably low. The known brake suffers from another problem that the output shaft becomes inoperable in the event of some failure of the electric motor, leading to an unsatisfactory level of fail-safe reliability of the brake.